The complexes translocon at the outer envelope membrane of chloroplasts (TOC) and translocon at the inner envelope membrane of chloroplasts (TIC) mediate preprotein translocation across the chloroplast outer and inner envelope membranes, respectively. Tic20, Tic56, Tic100, and Tic214 form a stable one-megadalton TIC whose function is essential in Arabidopsis thaliana and Chlamydomonas reinhardtii. Tic20 plays a central role in preprotein translocation by forming a protein-conducting channel. Tic56, Tic100, and Tic214 are also indispensable for TIC function, but whether other components are required for this process remains unclear. Here, we demonstrate that a 12-kD protein that we named Tic12 is part of the TIC in Arabidopsis thaliana and participates in preprotein translocation across the inner envelope membrane. Tic12 was tightly associated with the TIC but disassociated under high-salt conditions in combination with Triton X-100. Site-specific UV crosslinking experiments revealed that Tic12 and Tic20 directly interact with the transit peptide of a translocating preprotein. The tic12 null mutants are albino and seedling-lethal, similar to other tic null mutants. Tic12 and Tic20 were also involved in preprotein translocation in (Pisum sativum) pea chloroplasts. Thus, Tic12 is an essential constituent that forms the functional core together with Tic20 in the one-megadalton TIC.

中文翻译：

---

Tic12，拟南芥叶绿体内包膜易位子的 12 kD 重要组成部分

叶绿体外膜易位 (TOC) 和叶绿体内膜易位 (TIC) 复合物分别介导前蛋白跨叶绿体外膜和内膜易位。Tic20、Tic56、Tic100 和 Tic214 形成稳定的百万道尔顿 TIC，其功能在拟南芥和莱茵衣藻中至关重要。Tic20 通过形成蛋白质传导通道在前蛋白易位中发挥核心作用。Tic56、Tic100和Tic214对于TIC功能也是不可或缺的，但该过程是否需要其他成分仍不清楚。在这里，我们证明了一种被我们命名为 Tic12 的 12 kD 蛋白是拟南芥中 TIC 的一部分，并参与跨内膜的前蛋白易位。Tic12 与 TIC 紧密相关，但在与 Triton X-100 结合的高盐条件下解离。位点特异性 UV 交联实验表明，Tic12 和 Tic20 直接与易位前蛋白的转运肽相互作用。tic12 无效突变体是白化和幼苗致死的，与其他 tic 无效突变体类似。Tic12 和 Tic20 也参与豌豆叶绿体中的前蛋白易位。因此，Tic12 是与 Tic20 一起构成百万道尔顿 TIC 功能核心的重要成分。